When a mold is used in an injection molding machine, a small size mold (sub-mold) is installed into a base mold. In this case, only small size molds are exchanged while a base mold is commonly used, to thereby match the mold with various kinds of products having different shapes. This enables a manufacturing cost for the mold to be reduced.
Conventionally, a rotary fastening mechanism, a clamp mechanism, etc. have been used to install the small size mold. Besides, according to Japanese Laid-Open Patent Publication of No. Sho-58-42100, a mold plate loaded with the sub-mold is pulled out of a support frame for the mold plate to exchange the sub-mold.
However, in the case of installing the sub-mold with the rotary fastening mechanism, the clamp mechanism, etc., the installation will take a long time, so that the working efficiency of the injection molding is much reduced. Further, since the installation work is more complex, it has been difficult to realize automation for this process.
On the other hand, in the case of exchanging the mold plate, since an exchanging operation is carried out while every mold plate loaded with the sub-mold is suspended by a crane or the like and the mold plate is fixed with screws, it is also a time-consuming and complicated task.
Further, a product eject mechanism is supplied when it is difficult to take the product out of a cavity. However, in a conventional manner, only one eject pin penetrating from a base mold through a changeable sub-mold is provided, and the eject pin is moved to the cavity side to perform a product-ejecting operation (for example, as described in Japanese Examined Published Patent Publication of No. Sho-58-42100)
In this manner, the position at which the eject pin is provided is unchangeable, and thus the ejecting operation must be carried out at this position at all times. Therefore, the position of ejection is sometimes unsuitable for the shape of the cavity of the changeable sub-mold, and thus it frequently occurs that the molded products are not completely ejected or are damaged.
In order to obtain an optimum position for the ejection, it is necessary to reform the base molds. The reforming of the base molds will result in an obstacle to the requirement for reducing the manufacturing cost of the mold. The result turns out contrary to the expectation of decreasing cost.